The present invention relates to a process for working up or removing the organic secondary components obtained in the preparation of dinitrotoluene (DNT) by the nitration of toluene. These secondary components are separated with the process water from the crude DNT. It is necessary to treat these organic materials in order to be able to send the process waste water for a biological work-up.
In the conventional processes for the preparation of dinitrotoluene (DNT) from toluene and a mixture of sulfuric and nitric acids (nitrating acid), the acidic reaction water is distilled off in the sulfuric acid concentration step, and alkaline and acidic wash water from the purification of the DNT, are obtained as waste waters. In addition to mononitrotoluene and dinitrotoluene, this process waste water contains nitration by-products such as mononitrocresols, dinitrocresols and trinitrocresols (hereafter referred to globally as nitrocresols), picric acid and nitrobenzoic acids. These substances have to be removed from the waste water because aromatic nitro compounds do not easily degrade in biological waste water treatment plants and have properties toxic to bacteria.
The current state of the art for the treatment of organic secondary components in the nitration of aromatic compounds is as follows:
U.S. Pat. No. 6,506,948 describes the work-up of wash water from the purification of DNT prepared from toluene and nitrating acid. The DNT is recovered from acidic and alkaline wash water, with the organic secondary components remaining in the alkaline aqueous phase. A work-up of these secondary components is not, however, described in U.S. Pat. No. 6,506,948. Rather, there is merely a general reference to a possible chemical pretreatment (oxidation) or physical pretreatment (adsorption) prior to discharge into a biological waste water treatment plant.
The treatment of nitrocresols, which are separated from the product stream in the alkaline DNT washing step, can be effected by oxidative degradation with nitric acid at elevated temperatures according to EP A1 0 962 446. As described therein, however, this requires its own additional process stage. Temperatures of up to 180° C. are necessary for this process stage. In addition, an aftertreatment by adsorption on activated charcoal or a corresponding work-up in a biological waste water treatment plant is still required.
For the degradation of nitro compounds in the alkaline wash water from the work-up of nitrobenzene, U.S. Pat. No. 5,232,605 describes treating this waste water with nitric acid at temperatures of up to 290° C. and at pressures of up to 130 bar. After this separate additional process step, the waste water can be sent to a biological waste water treatment plant.
In the context of the preparation of nitrobenzene, U.S. Pat. No. 4,230,567 also describes the degradation of nitrophenols in an additional process step at elevated pressure and temperature. After alkaline washing of the nitrobenzene, the wash water is exposed to temperatures of 150° C. to 500° C. at pressures of 50 to 350 bar under an inert gas atmosphere.
After alkaline washing of the DNT, the process of U.S. Pat. No. 4,597,875 requires acidic precipitation of the nitrocresol components from the wash water and their mechanical settling. Then, the nitrocresol compounds are burnt in a suitable combustion process. Here again, however, an additional process step is required for working up the nitrocresol components.
The object of the present invention is to provide a simple and economic process for the separation and treatment of unwanted secondary components of the nitration. Surprisingly, this is achieved by means of steps that are simple in terms of process technology, particularly when compared with existing processes, and without additional process steps and feed materials that are foreign to the nitration process.